[runtime] QubitPlacer

[mpharrigan]

This PR defines a QubitPlacer abstract interface, some example concrete implementations, hooks into cg.workflow.execute to plumb in the new interface, and shims highlighting some of the blockers.

Including qubit placement as part of the quantum runtime is important for our benchmarking / data science requirements.

• We want fixed (frozen, immutable) executable: “the loschmidt echo benchmark”.
• Picking qubits is how you run, not what you run.
• Devices change, qubits change. We want to compare.

In order of increasing sophistication, I want to support the following strategies for qubit placement

1. Fallback: use circuit qubits: NaiveQubitPlacer
2. Explicit, by hand, per {topology x device}: HardcodedQubitPlacer
3. Random subgraph monomorphism: RandomDevicePlacer
4. Do a characterization to guide placement (not in this PR)

Please take a look at this sketch of the interface and the example concrete implementations given the above list. Please raise any structural or naming concerns now. I'll split up into a PR for the interface and runtime execute hooks, then one for each strategy.

You can see that there are several device-related blockers in this

1. I need a method to get a networkx graph from a device
2. Devices should be json serializable so we can record them in our SharedRuntimeInformation.
3. Devices should support __eq__ equality testing.

cc @MichaelBroughton @dstrain115 @tanujkhattar @dabacon

[mpharrigan]

This has been merged as part of #5194 #4701 #4700 #4719